Potential distributing insulator tube and method for making same



Nov. 4, 194.1. 1 w. sPooNER POQTENTIAL DISTRIBUTING INSLATOR TUBE ANDMETHOD OF MAKING SAME Filed May 14, 1940 l Inventor. Laurence W. 5

pooner;

by lA/7.6

l-las Attorn ey.

vconnecting the two electrodes.

Patented Nov. 4, 1941 UNITED STATE s PATENT OFFICE POTENTIALDISTRIBUTING .INSULATGB TUBE AND SAME METHOD Fon MAKING -Laurence W.Spooner, Pittsfield, Mass., assigner to General Electric Company, acorporation oi New York This invention relates to insulator tubes andmore particularly to such tubes having means for distributing apotential over the ytubes be ning arrester of this type comprising apair of tightly telescoped insulating tubes I and II having a lineelectrode I2 including a depending tween electrodes at the opposite endsthereof,

and a method for ymaking the same.

With certain types of insulators it is desirable to distribute thepotential difference between the electrodes at the opposite ends thereofover the length of the insulator for various reasons, such as, vtominimize/high potential gradients at the edges of one or the other ofthe electrodes or both. In certain forms of insulators such grading ofthe potential difference may be accomplished by means oi an element orcoating of semiconductive, high resistance material provided on theouter surface of the insulator and Frequently, as in the case of theinsulator tubes of arc expulsion type lightning 'arresters, theresistivity of the elementv must be relatively accurately determined andapplied in such a manner that it will be substantially indestructibleand weatherproof. The construction of such tubes by prior known methodshas been an inaccurate and expensive procedure. v

portion I3 threaded onto the upper end thereof and a ground electrode I4threaded onto the lower end thereof.

sume a potential determined b y the capacity be` tween this electrodeand the high voltage circuit and the capacity between this electrode andthe grounded electrode and any other nearbyk grounded object. The lineelectrode is usually much nearer the protected high voltage line than itis to the grounded electrode or any other It is an object of the presentinvention to provide a new and improved method of making an insulatortube having aA resistance potential distributing element connecting theelectrodes at the opposite ends thereof which method may be carried outin a simple manner and with a resultant high degree of accuracy in thefinished product. I

For a better understanding of my invention, attention is directed to thefollowing specification and the claims appended thereto taken inconnection with the accompanying drawing in which Fig. l is a crosssectional view of alightning arrester embodying an insulating tubeconstructed in accordance with my invention;.Figs. 2, 3, and iillustrate various steps in the manufacture of the insulating tube; andFigs. 5 and 6 are enlarged sectional views of opposite end portions' ofthe insulating tube illustrated in Fig. l.

The invention will be described as it may be applied to the constructionof a lightning arrester of the arc expulsion type though it will bereadily 'understood that the invention is not necessarily so limited. InFig. l is shown a lightgrounded object so that it tends to assume avoltage very much above the grounded electrode. The distribution of thishigh voltage along the insulating tube will not be uniform so that highpotential gradients and corona occur at the extremities of the lineelectrode parts. The .formation of corona during normal voltageconditions at the surface of the inner electrode part I3 not onlyresults in a deterioration of the tube and a shortening of its usefullife but also in considerable radio interference. To minimize theseeffects it is desirable to distribute the potential substantiallyuniformly over the length of the tube, which may be accomplished by theprovision of a resistance element or coating of a suitablesemiconductive material extending the length of the insulating tube andconnecting the opposite electrodes. A lightning arrester provided withsuch a high resistance coating is more fully described and claimed inthe copending application Serial No. 290,855 of Elmer J.Wade,

led August 18, 1939,-and assigned to the Cien-u eral Electric Company,the assignee of the presn ent application.

In order to-provide for the proper operation tivity which, forexample,may be of the order of about one-quarter to one megohm per inch.According to one method used heretofore the resistance element consistsof a coating of a semiconductive paint sprayed onto the outer surface ofthe insulating tube which had been previously suitably prepared. Thecoated tube is then f baked to harden the coating. Because of thefragility of this coating it must be further protected against injurydue to handling and weathering by an additional covering. First a layerof a suitable phenolic resin varnish is applied over the coatingfollowed by a baking step and then a few layers of phenolic resinvarnish impregnated paper are wrapped around the tube followed byadditional baking. It will be readily appreciated that the resistivityof the coating is influenced by each subsequent operation so that it isdiicult to arrive at a predetermined resistance value in the finishedproduct since minor variations in the different steps produce relativelywide fluctuations in resistance of the coating from one unit to thenext. The spraying of the resistance paint onto -the tube surface is avery difficult step to control with a high degree of accuracyl becauseof the many variable factors entering into this operation coupled withthe fact that few close checks can be made until each tube is fullycompleted and the final resistance of the coating measured. With thisprocedure spoilage due to faulty resistance values is excessively highresulting in a high cost for those found acceptable. I

In accordance with this invention I provide a plurality of wound layersIS of insulating material around the outer tube having a longitudinalstrip of semi-conductive high resistance material wound up therewithextending spirally or helically around the tube with the opposite endsthereof conductively connected to the electrodes at the opposite end ofthe tube. As illustrated in Fig. 2 a cylinder 2|, which iu. thisinstance may comprise the telescoped fiber tubes and for a lightningarrester, is supported upon a mandrel 22 adapted to be mounted within alathe or other suitable winding machine (not shown). One end of a web 23of suitable insulating material such as paper or cloth impregnated witha phenolic resin varnish may be attached to the cylinder 2| and bewrapped thereon by rotation of the mandrel 22. A resistance. element 24which has been previously treated and measured to have a predeterminedend to end resistance is placed upon the web 23 angularly with respectto the opposite edges'thereof so that only predetermined end portions ofthe resistance element extendbeyond the edges of the web. As the web iswound on the cylinder 2| the resistance element 24 will assume a spiralconfiguration around the tube between 'consecutive layers of the webmaterial.

After the element 24 has been completely wound into the tube and with adesired num- 2,2o1,ess

application. Such a tape can be readily manufactured with uniformity inlong lengths and measured portions can be cut off having a desiredpredetermined end to end resistance value. Defective portions of thetape can thus be readily detected and discarded before they are woundinto the insulator tube. Furthermore, minor variations of theresistivity of the tape portions can be compensated for by varying theeffective lengths of the strips by simply altering the angle at whichthey are laid across the web 23 before they are wound into the tubes.While I prefer to wind the element into the tube with the strip placedupon the web substantially at a degree angle in order to minimize theformation of voids at the edges of the tape, it will be readilyappreciated that this angle is not at all critical.

While the resistivity of the elements may be varied somewhat by thewrapping and subsequent baking of the wrapped tube, this is usually arelatively constant variation and may be readily compensated for in themeasurement of the resistance element prior to its being wound into thetube. v

It is obvious that insulator tubes having high resistance elementsembodied therein connecting the electrodes at the opposite ends thereofcan be manufactured in accordance with this invention with a high degreeof accuracy, in an expedient manner, and at a relatively low cost.

Having described an insulator tube and the A method for manufacturing itas adapted for one specific application, it is obvious that theinvention is not so limited and that it may be carried out in otherembodiments.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

l. A potential distributing insulating tube hav ing electrodes at theopposite ends thereof, said tube comprising a plurality of wound layersof insulating material, a semiconductive strip having a predeterminedresistance being wound into said tube between consecutive layers, theopposite ends of said strip being conductively connected to saidelectrodes.

2. A lightning arresterl including an insulating tube having anelectrode at each end thereof,

`said tube comprising a plurality of wound layers ber of additionallayers of wrapping applied, the

tube may be baked and then removed from the mandrel. Following this, theends of the tube may be suitably machined and threaded, as indicated inFig. 4, for cooperatively receiving the electrode parts |2 and Il. Itwill be noted that the opposite ends of the resistance element 24 areleft exposed by'the machining operation as indicated at 26 and to insurea good conductive connection between this edge and the electrodes, Iprefer to paint the beveled portions 21 as well as the threaded portions28 with a continuous layer 29 of conductive paint as indicated in Figs.5 and 6.

While the resistance elements or strips 2| may comprise any suitableflat material having a predetermined resistivity, I prefer to use acloth tape treated with a semiconductive mixture of unplasticizedcellulose acetate, colloidal carbon and solvatone such as is fullydescribed in my copending application Serial No. 305,933 led November24, 1939, and assigned to the General v Electric Company, the assigneeof the present of insulating material, a high resistance element woundspirally around said tube between consecutive layers of said insulatingmaterial, the opposite ends of said element conductively connected tosaid end electrodes.

3. In a lightning arrester including an insulating tube, an electrode ateach end of said tube, the outer surface of said tube being wrapped witha plurality of layers of insulating material, a relatively narrowlongitudinal semi-conductive strip having a predetermined resistancebeing wound spirally around said tube between said layers, the oppositeends of said strip being conductively connected to said end electrodes.

4. A lightning arrester comprising an insulating tube, an electrode ateach end of said tube, the outer surface of said tube being wrapped witha plurality of layers of insulating material, a semi-conductive strip ofmaterial having a predetermined end to end resistance being wound inbetween said layers of insulating material and conductively connected tosaid two electrodes.

5. A potential distributing insulating tube havmg electrodes at theopposite ends thereof, said tube comprising a plurality of wound layersof insulating material, a flat longitudinal strip of fibrous materialtreated with a semiconductive mixture and having a predeterminedresistance between spaced points thereof, said strip being wound intosaid tube between consecutive layers of lsaid insulating material withsaid spaced points at the opposite ends of said tube, said spaced pointsbeing conductively connected to adjacent electrodes.

6. A potential distributing insulating tube having electrodes at theopposite ends thereof, said tube comprising a plurality of wound layersof insulating material, a flat longitudinal strip of iibrous materialtreated with a semiconductive mixture and having a predeterminedresistivity, said strip being wound into said tube between consecutivelayers of said insulating material, said strip being exposed at theopposite ends of said tube and means conductively connecting theopposite ends of said strip to adjacent electrodes on said tube.

7. A potential distributing insulating tube having electrodes at theopposite ends thereof, said tube comprising a plurality of wound layersof insulating material, a fiat longitudinal strip of fibrous materialof-such dimensions and treated with such a semi-conductive mixture as toproduce a desired and predetermined resistivity, said strip being woundinto said tube between layers of said insulating material, said stripbeing ex- -posed at the opposite ends of said tube and meansconductively connecting the opposite ends of said strip to adjacentelectrodes on said tube.

f ing material and having a resistance element im- 8.' In a high voltagearc expulsion lightning arrester having a pairv of spaced electrodes, apotential distributing insulating tube for supporting one of said pair'of electrodes in each end :z

thereof, said insulating tube comprising an inner portion and an outerportion, said outer portion having a semi-conductive high resistancemeans embedded therein and connecting said pair of tributing insulatingtube from a web of insulatbedded therein for connecting electrodes atthe opposite ends thereof, said method including the steps of placing astrip having a predetermined end to end resistance angularly across saidweb with the opposite ends of said length positioned at the oppositeedges of said web, winding said web into a tube with the opposite endsof said strip exposed at the corresponding ends of said tube forconnection to adjacent electrodes.

10. A method of manufacturing a potential distributing insulating tubefrom a web of insulating material and having a resistance elementimbedded therein for connecting electrodes at the opposite ends thereoffor distributing a voltage over said tube between said electrodes, saidmethod including the steps of measuring a strip of semi-conductivematerial having a predetermined end to end resistance, placing saidstrip angularly across said web with the opposite ends of said stripprojecting slightly from the opposite edges of said web, winding saidweb into a tube, coating the ends of said tubeincluding the ends of saidstrip with a conducting material connecting the ends of said strip toadjacent electrodes. 4

11. A method of manufacturing a potential distributing insulating tubefrom a web of insulating material and having a resistance elementimbedded therein for connecting'electrodes at the opposite ends thereof,said method including the steps of measuring a fibrous strip coated witha semi-conductive material and having a lpredetermined end to endresistance, placing' said strip angularly across said web with theopposite ends thereof v positioned adjacent the opposite edgesv of saidweb, winding said web into a tube, m'achining the opposite ends of saidtube so as to expose the opposite ends of said strip, .coating theopposite ends of said tube with a conductive material for conductivelyconnecting the ends of said strip to adjacent electrodes.

LAURENCE W. SPOONER.

